Power-gas generating device



y 5, 1953 J. TAYLOR ETAL POWER-GAS GENERATING DEVICE Filed NOV. 4, 1947 N 0 m H v a N u WWO m n Y A A 3 a LG m RSTA H WEN r N c E VJRAW N Es I D m VA E L A Patented May 5, 1953 UNITE STT S TET OFFICE POWER-GAS GENERATING DEVICE assignors to Imperial Chemical Industries Limited, a corporation of Great Britain Application November 4, 1947, Serial No. 784,070 In Great Britain November 29, 1946 10 Claims. I

The present invention relates to new or improved power gas generating devices adapted to provide a stream of power gases at a predetermined pressure for a predetermined time from solid chemical compositions capable under suit.- able conditions of propagating through themselves without detonation a self-sustained exothermic gas producing decomposition.

The object of the present invention is to provide devices as aforesaid which are adapted to supply such a stream of power gases for considerable periods of time i. e. of the order of minutes, say from about /4 minute onwards and which are more convenient to produce and/or are more advantageous in action than those heretofore proposed.

Power gas generating devices according to the present invention are applicable for example for such purposes as the actuation of reciprocating or turbine engines or the propulsion of jet propelled apparatus or projectiles.

It has now been ascertained that it is possible to provide devices of the aforesaid kind containing pressed charges of ammonium nitrate or guanidine nitrate or nitroguanidine or nitro dicyandiamidine or dicyandiamidine nitrate or mixtures thereof sensitized with a powder sensitizer or catalyst so that the thermal decomposition of these compounds is caused to be selfsustained. For example, in such devices there can be included pressed charges of ammonium nitrate sensitized with a chromate or bi-chromate, which charges on the thermal decomposition of the ammonium nitrate lead to the propagation of gases consisting mainly of nitrogen, steam and oxides of nitrogen, the temperature of these gases being low i. e. of the order of 300 to 400 C. when a fuel is absent. By the inclusion of a suitable fuel adapted to consume more or less of the available oxygen, for instance a for the purpose of the present invention. Such charges are disclosed in British Patent No. 627,724 (corresponding to copending U. S. application Ser. No. 101,273, now Patent No. 2,604,391), and in British Pa tent No. 627,753.

According to the present invention the power gas generating devices adapted to provide a stream of power gases at a predetermined pressure for a predetermined time of the order of minutes are characterized in that the longitudinal surface of a compressed column of a powder mixture which includes at least one nitrogen base nitrate or nitro derivative whose thermal decomposition can be sensitized so as to become selfsustained and a powdered sensitizer of the thermal decomposition of said compound is in intimate contact with the internal surface of a protective structure adapted to prevent the decomposition of the sensitized composition from proceeding along the said longitudinal surface at a faster rate than that at which the decomposition of an exposed transverse surface proceeds through the said column.

The devices of the present invention can be conveniently produced in the form of greater diameter than a fewinches; the thermal decomposition of the pressed charges in the devices can be made to take place at relatively low temperatures; the yield of ash can be small; there is no tendency for any irregular decomposition to take place during the progressive thermal decomposition of the pressed charges in the devices; the composition of the pressed charges in the devices can be such that their thermal decomposition can be made to take place at normal. atmospheric pressures; and the temperature of the gases evolved on the thermal decomposition of the pressed charges can be as low as 300 to 400 C.

For ammonium nitrate compositions it is preferable to include a compound efiectively inhibiting the volume change that takes place at 32 0. It may also be desirable to include a fuel.

The column of the powder mixture is to be compressed at a pressure greater than the pressure at which the gas is to be generated and preferably of the order of 10 times this pressure. This compressing has the effect, of course, of compacting the powder mixture within a much smaller volume than it would normally occupy in its protective container and the residual elasticity oi the resulting compacted column ensures maintenance of the necessary degree of intimate contact between the sensitized charge and its protective container structure.

The internal surface of the aforesaid protective structure preferably consists of a layer or lining of heat insulating material so that the longitudinal surface of the compressed column of the powder mixture is in contact with the said layer of heat insulatingniaterial.

If desired the protective structure may also include a transversely disposed layer of heat insulating material or other protective material against which one end of the column of sensitized composition is compressed, e. g. for the purpose of protecting metal parts of the device from chemical attack by the sensitized composition or preventing decomposition from starting at that end of the column. If desired the said transversely disposed layer of heat insulating I material or other protective material may adhere to the said end of the column of sensitized composition.

The protective structure preferably comprises in addition to the layer or layers of heat insulating material in immediate contact with the column of sensitized composition an outer reinforcing structure of tough rigid material such as a metal container or casing, which facilitates the removal of the device from the press and enables pressure deformable materials to be used as the insulating layer or layers, and afford mechanical protection to the structure.

The linear rate of decomposition of the protected column ata given pressure may be controlled by including varying proportions of inert ingredients in the mixture, e. g. china clay, kieselguhr or potassium chloride; or by building up a column of graded composition by pressing successive increments of its length made up of differing individual compositions; or by modifying the geometrical form of the column. For instance, if the column is in the form of a, solid cylinder it will have a constant burning surface, but if it is provided with a coaxial channel so that it takes the form of an annulus of which only the external cylindrical surface is protected, its exposed surface will continually change in area as the reaction proceeds. By such expedients a desired working pressure may be maintained if the rate of consumption of the gases in the apparatus increases or decreases with the time of its action.

As the insulating material in contact with which the powder mixture is compressed, there may be used any suitable material able to fulfill its function of preventing decomposition along the peripheral surface of the column ahead of the exposed transverse surface, and it will usually be necessary that this should be reinforced by a metal cylinder. The insulating material must not react with the solid charge, e. g. in the case of sensitized oxygen positive nitrate charges it must be a substantially incombustible material. For instance even with oxygen positive compositions a sheet of Hallite, which is a rubber bonded asbestos sheet material containing about 80% asbestos, supported on brass wirecloth, may be used in conjunction with a metal container. The insulating material may or may not be appreciably susceptible of permanent deformation on compression.

In the case more especially of devices comprising an unchannelled column of sensitized ammonium nitrate compositions, the column and. its insulation have a certain tendency to partfrom the surrounding metal reinforcement if the: structures are exposed to temperatures substan-- tially in excess of the temperature at which they were formed in the pressing operation and are again allowed to cool to the lower temp atu This is an effect due to further compression of the column occasioned by its having a higher temperature coefiicient of linear expansion than that of the metal reinforcement. In the case of devices comprising a column of sensitized composition of large cross sectional area a separation of the metal reinforcement from the insulated column may take place as a result of exposure to such temperature changes, and in the case of some heat insulating materials the adhesion between the heat insulating layer and the compressed column of sensitized composition would not be sufficiently good to prevent its detachment from the column. Any danger of detachment in such circumstances may be obviated by employing as the heat insulating layer a pulverulent composition compressed into compact form around the sensitized composition against the metal reinforcement by the pressing operation in forming the device, this pulverulent composition preferably comprising an inert ingredient such as china clay and if desired also the characteristic gas generating thermally decomposable compound present in the composition forming the column, without including any of the sensitizer, but in the case of ammonium nitrate compositions including an inhibitor of volume change at the 32 C. transition point. When such a composition is used the ingredients of the insulating layer and the sensitized column to some extent intermix and when compressed together against the metal reinforcement they become inseparably bound together.

When the column of compressed sensitized material is required to have a channel, the channel may be formed by drilling it out.

As the pulverulent sensitizer of thermal decomposition of the nitrate or nitro compound of the nitrogen base there may be employed for instance in the case of ammonium nitrate; a chromate; bichromate o-r polychromate of an alkali metal or ammonium, e. g. potassium chromate, potassium bichromate or ammonium bichromate or mixtures of these; a hypophosphite of ammonium or of an alkali metal, alkaline earth metal, e. g. sodium, potassium, barium hypophosphite, manganese dioxide, ammonium thiosulphate, copper chromite, or nickel chromite; Mixtures of sensitizers may be used in many cases.

The volume change due to the transition of ammonium nitrate at 32 C. is most effectively inhibited by potassium salts, which may or may not be compounds capable of sensitizing the thermal decomposition of ammoniumnitrate. As the compound inhibiting the volume change due to the transition of ammonium nitrate that occurs at 32 C. in the absence of the compound there may be used for instance potassium nitrate, preferably in amount 10% of the weight of ammonium nitrate, but other potassium salts capable of reacting with ammonium nitrate by metathesis may be used instead. Potassium chromate or bichromate may for instance be used. It is advisable to subject the mixture of the ammonium nitrate and the potassium salt used to a short period of storage above 35 C. before the mixture is compressed.

As pulverulent sensitizers for nitroguanidine 0r guanidine nitrate there may be used for in stance chromic oxide, a chromate, dichromate or polychromate of the alkali metals or ammonium; hypophosphites of ammonium, of alkali metals or of barium; copper powder and a number of copper compounds as for instance cuprous oxide, cuprous chloride, cuprous oxalate, cupric ch10:

ride, cupric oxide; a number of nickel, silver, tin, selenium and vanadium compounds.

As further pulverulent sensitizers or the thermal decomposition available in the case of nitroguanidine there may be mentioned for example a number of salts of the alkali metals, e. g. potassium nitrite, sodium nitrite, potassium carbonate, sodium carbonate (salts of potassium being more active than those of sodium) a number of aluminium compounds, e. g. aluminium chloride, aluminium oxide; various compounds of lead, molybdenum or zinc, for instance lead nitrate, molybdic acid, zinc metal, zinc oxide and zinc carbonate, the two last mentioned compounds being especially active sensitizers.

For dicyandiamidine nitrate and nitrodicyandiamidine nitrate the pulverulent sensitizer may be for instance a bichromate or a hypophosphite of an alkali metal or ammonium.

It will be understood that combinations of different sensitizers are. often desirable to use, While also it may be advantageous to use mixtures of the nitrates and/or nitro compounds of nitrogen bases.

The sensitized compositions made from ammonium nitrate may if desired include an organic fuel capable of undergoing oxidation during the thermal decomposition of the ammonium nitrate, for instance an organic compound such as anhydrous ammonium oxalate. The tendency is for the calorific value of the composition to increase to a maximum as the amount of such fuel is increased sufficiently to consume the available oxygen with the formation of carbon dioxide and water, and then to fall with further increase in the content of the fuel. The temperature and nature of the gases produced and the rate of decomposition may thus be varied by controlling the oxidizable fuel content of the composition. Usually the cooler compositions i. e. oxygen positive or strongly oxygen negative compositions are preferable for operating engines, whereas the hotter compositions are desirable for propulsion by gas escape reaction.

As in the case of sensitized ammonium nitrate, the temperature at which the sensitized nitrates and nitro derivatives of the organic nitrogen bases decompose are substantially lower than those yielded by the burning of black powderlike compositions or smokeless powder, but whereas ammonium nitrate is oxygen positive these compounds are oxygen negative. The inclusion of oxygen negative organic compounds thus tends to lower the temperature at which the gases are produced from them. Thus with sensitized nitroguanidine further reduction of the temperature of the gases evolved on the progressive reaction of the mass of the highly compressed homogeneous powder composition can be achieved by including up to 10% of dimethyl cliphenyl urea which is of high carbon content 70%? and low oxygen content (3.5%). Other suitable additions by way of example are starch or other cellulosic compounds. Although the rate of combustion is slowed down by the addition of dimethyl diphenyl urea, the stability of the mass during its burning is much improved and steady burning without the breaking up of the charge is obtained at burning pressures up to at least 220 lbs. per square inch.

As the gases evolved may be contaminated with the said sensitizer or its decomposition products as for instance green chromium oxide, in a fine state of subdivision precautions may have to be taken to see that all such finely divided 6 material is removed before the'gases can be used to drive an engine.

The invention is illustrated in the accompanying drawings, in which:

Figs. 1, 2 and 4 are sectional views illustrating diagrammatically several typical forms of power gas generating devices of the present invention, and

Fig. 3 is a sectional view of part of the device shown in Fig. 2, in which the compressed powder charge assumes a slightly modified form.

In order that the invention may be more easily understood, a number of specific examples embodying the invention are given hereinafter. In these examples, the proportions of the various components of the gas-producing charges are in parts by weight. It is to be understood, however, that these examples are merely illustrative of the broad inventive concept, and are not to be construed as limiting the invention thereto.

Example I A gas generating device, as shown in Fig. 1, suitable for actuating a William and James motor ill, which is a Zr-cylinder reciprocating engine. consisted of a strong walled steel tube H of 4.7 inches internal diameter and 5 inches external diameter closed at one end and provided with a lining of l-iallite steam jointing sheet material it? into which was'pr'essed in increments under a pressure of 5,550 lbs. per square inch, to form a continuous column, 15 lbs. of a ballmilled powder composition [3 consisting of:

Parts by weight Ammoniumv nitrate 78.5

Potassium nitrate 9.0 Anhydrous ammonium oxalate 6.9 Ammonium bichromate 5.6 China clay 0.7

To test the suitability of the device for itsv purpose the open end of the tube was closed by a head Ma and coupled by a steel pipe i4, 6" long, to a William and James motor designed for starting bus engines. The charge which is about 15" long. was ignited at its free end by means of an electric powder fuse 5 containing 5 grains black powder, and a disc of black powcler primed cambric, and the gases were filtered through a slag Wool filter Hi introduced into the open end. of the steel tube. The decomposition of the charge caused the engine to run for 3 minutes 20 seconds. The pressure rose to a steady maximum of 280 lbs. per square inch and the engine developed an average brake horse power of and a maximum brake horse power of 4.1. The temperature of the gases leaving the device was 609 C. while the temperature at the inlet to the engine was 349 0., the drop in temperature being due to heat losses in the connecting tube. The engine ran smoothly throughout the test, and when examined afterwards was found to be in good condition and free from corrosion.

Example II Referring to Fig. 2, the propellant composition employed was composed of the following components:

Parts by weight Ammonium nitrate 78.5

Potassium nitrate 9:3 Ammonium dichromate 5.6 Ammonium oxalate (anhydrous) 6.9 China clay 1.0

149 lbs. of the above composition were prepared were by mixing the ingredients -in a ball mill and were then pressed into a 17 inch diameter iron pot l1 lined on sides and base with Hallite jointing sheet I2. The mixture was pressed into the iron pot in 6 equal increments, each being compressed under a load of 12,000 lbs. per square inch to form the propellant charge l8. A charge of 8 lbs. of china clay was spread evenly over the base of the pot on top of the Hallite lining before loading the first increment. When pressed the china clay forms a firm incombustible layer I9 bonded to the combustible charge l8 thus ensuring even burning during the last stages of combustion by supporting the charge when it reaches the form of a thin circular plate.

The charge was placed on its side in a massive decomposition chamber 20 which was connected directly by means of high pressure piping M to a four cylinder compressed gas operated reciprocating engine Hi. The charge was ignited by means of a fuse containing a small quantity of black powder so placed that flames were caused to play on the surface of the charge. The gases from the charge were passed through a filter 22 containing slag wool before being fed to the engine.

The engine was driven by the decomposition gases for 2 minutes at an average pressure of 530 lbs. per square inch and the engine developed an average power of 150 B. I-I.P.

Jet with similar charge-A device including a charge having the same composition as above and prepared in the same way but containing only 120 lbs. of the active material was loaded in the same firing chamber as that shown in Fig. 2 and ignited in the same manner. The gases, instead of being passed to an engine through a filter were vented directly to the atmosphere through a nozzle of circular cross-section with a diameter of 0.53". The charge burned at a constant pressure of 390 lbs. per square inch for 1 minute 40 seconds in which time the charge was completely consumed.

Example III A slightly modified construction of the propellent charge employed in Example II is shown in Fig. 3, the charge composition, however, being the same as in Example II. Into a 17 inch internal diameter iron pot Was introduced an end disc 23 of Hallite just fitting therein and resting thereon a 3 2" Hallite sheet lining 24 for the metal pot IT. A bottom layer IQ of 8 lbs. china clay was next introduced. A thin metal cylinder 16 /2 inches in diameter open at both ends was then introduced coaxially with the pot, and into this was introduced a measured quantity of the ball-milled composition comprising the sensitized ammonium nitrate making up the main body 25 of the propellant charge. Between the thin metal cylinder and the "Hallite lining was introduced a ball-milled powder made from ammonium nitrate 36%, potassium nitrate 4%, china clay 60%, to form an annulus 26 of this material adjacent the wall of pot I'l, having the same height as and completely surrounding the main body 25 of the sensitized composition. The thin metal tube was then withdrawn and the material pressed in place. This operation was repeated for each increment of pressure, of which there were six, each at 12,000 lbs. per square inch, so that the compressed charge became bound to the lining of inactive ammonium nitrate composition during the application of the pressure. When the pot containing the charge was stored at 60 C. and allowed to cool the Hallite sepa- 8 rated from the metal but cohesion between the active and the inactive compositions remained perfect.

Ewample IV A composite structure similar to that shown in Fig. 1 consists of a strong walled steel tube of 4.7" internal diameter lined with e 5" thickness of Hallite and 5.0" external diameter contain-. ing therein 8 lbs. of a powdery composition pressed under a pressure of 6,000 lbs. per square inch so as to give a compressed charge about 9 inches long, the said powdery composition being ball-milled and consisting of:

Parts Nitroguanidine 56 Guanidine nitrate 28 Ammonium bichromate 8 Dimethyl diphenyl urea 4 Beech charcoal 4 To test the suitability of the composite structure for the production of gases on the burning of its compressed charge, the steel tube Was coupled by steel pipe 6 feet long to a William and James motor designed for starting bus engines. The charge was ignited at its uncovered end by means of an electric powder fuse containing 5 grains blackpowder, and a circular disc of black powder primed cambric. The gases were filtered through a steel wool filter incorporated in the steel tube. The engine ran for 3 minutes 20 seconds at a pressure rising slowly from to lbs. per square inch and developed 1 to 2 HP. The temperature of the gases leaving the charge tube was found to be 600 C., while the temperature at the inlet to the engine was 340 C., the temperature drop being due to heat losses in the connecting tube. The engine ran smoothly throughout the test and when examined afterwards was found to be in good condition and free from corrosion.

Example V A composite structure according to the invention consists of a steel tube of 4.7" internal diameter and 5.0" external diameter closed at one end and provided with a lining of a rubber contain 'ng asbestos-graphite steam jointing sheet and containing therein 5 lbs. of a powdery composition pressed under a pressure of 5,500 lbs. per square inch the said powdery compositions being ball-milled and consisting of:

Parts Guanidine nitrate 94.5 Vanadium pentoxide 0.5 Cuprous oxide 5.0

Example VI For the device shown in Fig. 4, 8 lbs. of a powdery composition consisting of:

Ammonium nitrate Potassium nitrate N 9 0 Ammonium oxalate (anhydrous) 6 9 Ammonium dichromate 5 6 China clay 2.5

were pressed in 8 equal increments on a base 27 of lb. china clay in a 3 /2 internal diameter steel tube 28 lined with Hallite. A one inch diameter hole 29 was then drilled down the center of the pressed composition to the china clay layer.

The charge was ignited in the central hole and on the top surface. Filtration was by means of an annular filter l6 packed with slag wool. The charge was connected by means of a 2" diameter pipe M to a small motor 30 of rotary blower design coupled to an electric dynamometer 3E. The pressure developed and the brake horse.

power figures are recorded in the following table:

Time (seconds) l0 .5 Pressure (lbs/sq. in.) 5O Brake horsepower i 6.5 .0 11.5 13.5

Example VII and the brake horsepower figures are recorded in the following table:

Time (seconds) 5 10 15 Pressures (lbs/sq. in.) i5 Brake horsepower 5 .8 13

The expression the nitrate and nitro derivatives of a nitrogen base compound employed in certain of the appended claims is to be under stood as sufiiciently generic to include ammonium nitrate.

We claim:

1. A power gas generating device adapted to provide a stream of power gases at predetermined pressure for a predetermined time on the order of minutes comprising, in combination. a. compressed continuous column of a sensitized thermally decomposable powder mixture confined within an externally rigid protective structure which includes an internal lining of heat insulating material substantially inert toward said powder mixture, with which lining the longitudinal surface of said column is in direct contact. said powder mixture including a thermally decomposable compound selected from the group consisting of the nitrate and nitro derivatives of a nitrogen base compound, and a powder sensitizer of the self-sustained thermal decomposition of said compound; said powder mixture having been compressed in situ in said protective structure at a unit pressure on the order of at least ten times that at which the gas is to be generated to provide a compressed column of compacted powder mixture whose longitudinal surface is maintained in intimate cohering contact with the internal heat insulating surface cf said protective structure, whereby decomposition of the sensitized powder mixture is prevented from proceeding along said longitudinal surface at a faster rate than that at which decomposition of an exposed transverse surface proceeds through said column.

2. A power gas generating device as claimed in claim 1, wherein the internal surface of said protective structure consists of a layer of heat insulating material composed largely of asbestos,

i0 withwhlchsaid powder mixture has been pressed into intimate contact. 4 I

3. A power-gas generating 'd'evic'e as claimed in claim 1", wherein said internal lining comprises a layer of substantially inert pulverulent composition bound to the surface of said column by the compression of said powder mixture and said insulating layer of pulverulent composition in situ within said structure.

4. A power gas generating device as claimed in claim 3, wherein the inert pulverulent composition includes said thermally decomposable composition but without any sensitizer- 5. A. power gas generating device as claimed in claim 3, wherein said inert pulverulent composition is preponderantly china clay.

6. A power gas generating device as claimed in claim 1, wherein said powder mixture includes ammonium nitrate, a sensitizer capable of effecting self-sustained thermal decomposition of said nitrate and a potassium salt which effectively inhibits the volume change of said nitrate occurring at 32 C.

7. A power gas generating device as claimed in claim 6, wherein the potassium salt is potassium nitrate.

8. A power gas generating device as claimed in claim 7, wherein the amount of potassium nitrate included is 10% on the weight of the ammonium nitrate.

:9. The method of preparing a power gas generating device adapted to provide a stream of power gases at a predetermined pressure for a predetermined time on the order of minutes which comprises placing within an externally rigid protective structure, which includes an internal lining of heat insulating material su stantially inert to the gas generating charge it is to contain, a thermally decomposable powder mixture including a compound selected from the group consisting of the nitrate and nitro derivatives of a nitrogen base compound, and a sensitizer capable of effecting self-sustaining, nondetonating, thermal decomposition or said mixture, and compressing said mixture into a solid compacted form within said protective struc ture at a unit pressure on the order olat least ten times that at which the power gas is to be generated, thereby causing the compacted powder mixture and said lining to cohere.

10. The method of preparing a power gas generating device adapted to provide a stream of power gases at a predetermined pressure for a predetermined time on the order oi minutes upon self-sustained thermal decomposition of a powder mixture, which comprises: placing a temporary, open-ended, columnar retainer within an outer permanent, rigid walled protective casing to provide an annular space between the two; filling said annular space with a pulverulent, heat-insulating, inert composition; filling said open-ended retainer with such thermally 8.600111" posable powder mixture; withdrawing said temporary retainer longitudinally of said permanent casing, and thereafter compressing said inert composition and said powder mixture in situ in said casing to bind said powder mixture and said inert composition together.

JAMES TAYLOR. ALEXANDER CANTLAY HUTCHISON. ISAAC FAGELSTON.

(References on following page) 11 References Cited in the file of this patent UNITED STATES PATENTS Number Number Name Date Lyman- June 30, 1885 Unge July 1'7, 1906 Bourdelies May 24, 1910 Wert Oct. 23, 1923 Winter Jan. 13, 1948 FOREIGN PATENTS Country Date Great Britain 1907 Number Country Date Great Britain Sept. 7, 1936 Great Britain July 20, 1938 Great Britain Mar. 11, 1942 Great Britain July 4, 1945 Great Britain June 27, 1945 France Feb. 24, 1920 France Nov. 18, 1946 OTHER REFERENCES Astronautics, vol. 19, page 6, May 1932. 

